The objective of this study is to evaluate the safety and efficacy of Lu 26-054 and citalopram in the treatment of panic disorder with or without agoraphobia. The clinical trial will be conducted as a randomized, double-blind, placebo-controlled, multicenter, parallel, flexible dose study. The study consists of a two-week single-blind placebo lead-in period, followed by a ten-week double-blind treatment period. Approximately 120 patients will be randomized to each double-blind treatment group (Lu 26-054, citalopram, and placebo), for a total of approximately 360 patients. Patients who meet eligibility criteria at the screening visit will enter a two-week single-blind placebo lead-in period. Patients who complete the placebo lead-in and continue to meet all entry criteria will be randomized to ten weeks of double-blind treatment with placebo, Lu 26-054, or citalopram. After the baseline visit at the end of the placebo lead-in, study visits will be conducted after 1,2,4,6,8, and 10 weeks of double-blind treatment. Patients who have missed two consecutive scheduled visits will be discontinued from the study and administered safety evaluations only.